Such a detection may for instance, but not exclusively be used by the production of plates based on wood and manufactured by a gluing together of wood particles of a varying size, i.e. plywood made of individual layers, OSB (Oriented Strand Boards) made of large strips of wood, chipboards (made of a mat of layers of large and small chips usually separated in layers), and fibreboards made of wood fibres.
By the manufacture of these plates, a thermosetting glue is applied onto the particles, said glue being laid out or scattered on plates or tapes in the form of a laminated or homogenous mat and subsequently subjected to a continuous or non-continuous process in a hot press so as to be pressed or cured.
The parameters of the process and especially the characteristics of the press have an effect on the typical properties of the completed plate. The latter is particularly obvious in connection with the density profile of the plate, i.e. the variation of the density over the thickness which accordingly is an indicator of both the operating conditions of the production plant and of the use properties of the plate.
According to the prior art it is possible to destructively determine the density profile on laboratory tests, either gravimetrically by way of a milling off and weighing in layers or by way of an isotope-transmission scanning on a test sample in the plane of the plate. Based on these results, the process can be adjusted, but not without involving a time-delay of at least 1 to 2 hours.
A demand exists for a possibility of performing a non-destructive, on-line-determination of the density profile in the plate in such a manner that it is possible to adjust the process very quickly, typically in a few minutes without interrupting said process and without involving a sampling and laboratory tests.
German Patent specification No. 4,243,454 discloses a way of measuring the density profile along the edge of a plate by means of a pencil of rays and a plurality of detectors arranged below the plate. The pencil of rays is obliquely emitted inwards from the side along the edge of the plate. A resulting advantage is that nothing but the attenuation in the lowermost layer is measured by means of a first portion of said pencil of rays, whereafter the attenuation in the lowermost and the lowermost but one layer is measured by means of another part of said pencil of rays, etc. Subsequently, it is possible to calculate the attenuation in each layer. This method is, however, encumbered with the draw-back that it is too unreliable. In addition, it only involves a measuring along the edge, which is not always sufficiently representative.
U.S. Pat. No. 5,195,116 discloses an apparatus for detecting the layers of separation in a laminated plate by means of X-rays. A narrow pencil of X-rays is scattered as the consequence of the compton-effect. A detector directed towards a measuring volume detects the radiation scattered therefrom, said radiation representing the electron density and consequently the density in the measuring volume. A displacement of the source and the detector, respectively, in an up and downward direction relative to the plate renders it possible to obtain information on the structure of the layers and on possible errors in the material. This measuring system is, however, encumbered with the draw-back that the signal and the scattered radiation intensity from a specific measuring volume beyond the density also depend on the thickness and density of all the superposed layers. These values can be determined in principle. The latter would, however, require extensive Calibration measurings on known articles. In addition, possible measuring errors are accumulated from all individual layers to the instant measuring volume.